In power plants, flue gases derived from fuel contain impurities, which are often oxides of sulphur (SOx) because of the sulphur compounds contained in the fuel. These flue gases are cleaned with a gas scrubber, i.e. an absorber, wherein the sulphur dioxide is removed with the washing fluid by using an absorbent in the washing fluid, which absorbent reacts with sulphur dioxide. Limestone-based scrubbers are based on that the sulphur oxides coming with the flue gases react with calcium carbonate and form calcium sulphite, which oxidizes to calcium sulphate (gypsum) under the influence of the oxygen contained in the flue gases and forced oxidation. After this the gypsum can be separated from the fluid and the fluid can be recycled back to the scrubber.
Limestone scrubbers are advantageous to use because of their low operating costs (cheap absorbent). However, problems have been caused by the detection of foaming and cumulation of the foam in the limestone scrubber in diesel power plants and in other power plants. With the flue gases in a diesel power plant come lubricants and soot, which together with the bubbles of the oxidation air easily form foam onto the fluid surface of the scrubber. Also, the quality of the water used in the washing fluid has an effect on foaming. Because of the recycling of the washing fluid, foam also continuously circulates back to the scrubber, because it does not exit via the hydrocyclone, which is used in separating gypsum from the washing fluid. Because of this, the amount of foam on the fluid surface of the scrubber increases.
In power industry, it is possible to use a conventional solution for the problem of foaming: the use of anti-foam agent. However, anti-foam agents cause the problem that they may disturb scrubbing process. In addition, their dosage must be continuous and they increase the operating costs of the scrubber. On the other hand, the removal of foam from the scrubber by adjusting the running sequence is difficult because of the floatability of the foam.